We're currently using the "base_ClassificationExtendedness" "CatalogCalculation" plugin as our primary star/galaxy classifier everywhere. This is motivated by evidence from SDSS that this classifier was about as good as a simple S/G classifier could get. We do not have similarly strong evidence for our own implementation of extendedness (or the CModel measurements on which it heavily depends) being as good.
More importantly, we're also using this classifier on single-visit catalogs in which CModel isn't being run, with GaussianFlux as the stand-in for the galaxy model flux. This has never been shown to be much good, aside from the general sense that we've been doing this for a long time and it's seemed.
But "extendedness" has long been known to be sensitive to aperture corrections, and we've known for a while that our aperture corrections are noisy and sometimes biased, leading to an effort to reduce our dependence on them and do them more carefully. And
DM-38733 has implicated extendedness in our problems with stellar-locus metric sensitivity.
So, it's long past time to replace our usage of extendedness on single-visit images with something better-motivated and ideally not dependent on aperture corrections, and the obvious solution is something that compares the source moments to the PSF moments. Doing that somewhat rigorously - using the uncertainties on the source moments to compute an actual likelihood - seems like a good start.
I think we should probably write this as a regular measurement plugin, not a CatalogCalculation; the latter was invented as a workaround for extendedness' dependency on aperture corrections (we first measure aperture corrections after running all regular plugins). Perhaps after we have a solid replacement, we can make extendedness into a regular plugin and drop the CatalogCalculation concept.